The Quantum Nature of C-H···Metal Interaction: Vibrational Spectra and Kinetic and Geometric Isotope Effects of Adsorbed Cyclohexane

摘要

The nature of C-H···M (M = metal surface) interactions is reviewed based mainly on our recent investigations of cyclohexane on Rh(111). Infrared reflection-absorption spectroscopy measurements at low temperature (～20 K) have shown that the softened CH stretching band consists of several sharp peaks. At temperatures above 80 K, each peak is broadened, most probably by anharmonic coupling with thermally excited low-energy frustrated translational modes. The origin of fine structure in this band and its similarity to that in hydrogen bond systems are discussed. In addition, novel isotope effects were observed in desorption kinetics and adsorption geometry of cyclohexane on Rh(111) using temperature programmed desorption, ultraviolet photoelectron spectroscopy, and spot profile analysis low-energy electron diffraction. The desorption energy of deuterated cyclohexane (C_6D_(12)) is lower than that of C_6H_(12) (inverse kinetic isotope effect). In addition, the work function change by adsorbed C_6D_(12) is smaller than that by adsorbed C_6H_(12). These results indicate that C_6D_(12) molecules are slightly more distant from the surface than C_6H_(12) molecules (vertical geometric isotope effect). A lateral geometric isotope effect was also observed for the two-dimensional cyclohexane superstructures as a result of the repulsive interaction between interfacial dipoles (= work function change). These isotope effects are ascribed to the quantum nature of hydrogen involved in the C-H···M interaction.